Why don't birds' feet freeze in the cold?
Sometimes it seems like you can catch a cold just from seeing a bird outside in the middle of winter. How does a coat of flimsy feathers keep sub-zero temperatures at bay? And those feet. Barefoot in winter? Seriously? The thing is, more than a few well-meaning ''humans take it a step further — and call local wildlife authorities, practically begging them to get that bird out of the cold. Chantal Theijn has been there. As founder of Hobbitstee Wildlife Refuge in Ontario, Canada, she has seen birds in a lot of dire situations. Barefoot in winter is generally not among them. "Very very rarely, I have seen it," she tells MNN. And only on those occasions that a flash freeze caught them by surprise. Like four years ago, when the Great Lakes iced over completely. "You know how they swim around in the water to keep water open?" Theijn explains. "Because the frost was so severe so fast they froze in the ice." But there's a very good reason why even the coldest of winters aren't much of a problem for most birds: Their feet are ingeniously designed so that they're already cold to begin with. Thanks to a network of arteries — called rete mirabile or "wonderful net" — a bird's heart is wired to its feet in such a way that by the time the tiny amount of blood gets down there, it's cooled. And when blood flows back up, it's warm. This heat exchange system ensures warm blood stays close to the bird's heart, while the cool stuff dribbles down to its toes. The bird feels very little down there, and, most importantly, doesn't experience any heat loss. A couple of biological tweaks make this system even more efficient. For one thing, a bird's arteries actually sink deeper into their bodies during winter, leaving them less exposed to the elements. And then there's the ace up the bird's feathered sleeve: no muscles at all on its lower legs and feet. That means they hardly need more than a ''pitter-patter of blood to do what they need to do. That's not to say a bird's foot couldn't use a tiny mitten now and then. In serious cold, birds use their whole feathered bodies as a mitten — explaining why you'll often seen them bunched up on the ground, keeping those little extremities toasty. And that's where humans can actually cause a problem. "If there's Canada geese sitting with their feet tucked up and people keep forcing them to move, then they can also get frostbite that way," Theijn explains. Another unfortunate way humans can mess up a bird's natural cold defenses also happens to be a way we mess up much of the world: chemical spills. "Most waterfowl can't swim. They float," she says. "If their feathers weren't waterproof, they would lose their ability to float and they would sink like a brick. "That's why oil is such a problem. Not only is oil toxic, but it affects the waterproofing of their feathers. And it causes them to get wet and sink essentially." And for all their cold-weather proofing, no bird survives the icy depths of a lake. The moral of this winter tale? Birds handle this weather just fine on their own. They only thing they have to worry about is us, unfortunately. Gallery: ' '